The present invention generally relates to the field of wastewater treatment and disposal and more particularly relates to an apparatus and method for disinfecting biosolids to facilitate their use in land farming and as a soil additive.
Sewage, or wastewater, produced from municipal, industrial, or agricultural generators is typically comprised of a quantity of solids generated from humans, animals, fish or other living organisms, commonly called xe2x80x9cbiosolidsxe2x80x9d, suspended in a quantity of water. Such biosolids, also known as sewage sludge, pose threats to the health and safety of the environment because of the harmful pathogens, bacteria and microorganisms that are typically present in such biosolids. It is recognized that processing the wastewater by separating the water from the biosolids and treating and re-using the water and the biosolids is of great value in extending the useful life of limited earth and water resources and in preventing the communication of diseases.
Water that results when the biosolids are separated from the wastewater is usually treated with chlorine as a chemical disinfectant and discharged into the nearest river or stream where it is ultimately returned to a water supply system for re-use. Treated biosolids are often disposed of by delivering the treated biosolids to a landfill, to land farms for spreading the treated biosolids over available land, by incineration, by disposal at sea or by use of the treated biosolids for agricultural purposes such as a soil additive. Regardless of the disposal method employed on the biosolids derived from the wastewater, any safe biosolids disposal method requires the elimination, or at least the sufficient inactivation, of the harmful pathogens, bacteria and other microorganisms present in the biosolids. Thus, some type of disinfecting method must be employed before the disposal or reuse of the biosolids.
A range of treatments can be applied to disinfect the biosolids. Such treatments include pasteurization (dry heat), aerobic (oxidative) or anaerobic digestion, composting, lime stabilization, liquid storage and dewatering and dry storage. None of these methods fully kills or deactivates all of the live microorganisms or the vegetative or spore-forming microorganisms typically present in biosolids.
Disinfecting sewage by continuous thermal processing employing two-stage steam heat has been proposed in U.S. Pat. No. 4,028,242 to Kokurin et al. The first stage of the process discussed in Kokurin et al. is a first nozzle employed for mixing sewage drawn from a holding tank with a supply of steam in a pressurized piping system. The second stage is introducing the steam/sewage mix to a pressurized pipe coil via a second nozzle and maintaining the flow through the coil at a desired temperature for a desired time by mechanical means to facilitate the disinfecting of the sewage. A disadvantage of the method presented is that the mechanical augers, called cyclones in Kokurin et al., employed in the pipe coil to circulate and disperse the flowing sewage to facilitate thorough heating of the sewage are complicated and may require frequent and costly maintenance. Still another disadvantage is that the system presented makes no provision to prevent heat loss from the pipe coil as the sewage is conveyed through the coil. As a result, a need still exists for an apparatus and method to thermally treat and disinfect a continuous flow of biosolids from a stream of wastewater, avoid where possible mechanical conveying means, and reduce heat losses associated with such thermal treatment.
The present invention is designed to provide an apparatus and method to facilitate the thorough thermal disinfecting of biosolids generated from a stream of wastewater. The apparatus and method employs a means for producing a continuous flow of biosolids derived from wastewater, by way of biosolids input piping and pumps, to a biosolids storage tank. A uniquely configured steam disinfecting piping system is arranged within the biosolids storage tank. A second pumping system delivers a continuous flow of biosolids to the steam disinfecting piping system. A steam generating means provides a continuous supply of steam, by way of steam piping and steam pumps, to the steam disinfecting piping system. The heat produced by circulating steam disinfects biosolids circulating through the steam disinfecting piping system. Recovered steam is returned to the steam generating means. Volatile gases produced from the biosolids during disinfecting are delivered to an incineration system. Disinfected biosolids are collected for delivery either to a landfill, an incinerator, or collected for use in land farming or as a soil additive.
The uniquely configured steam disinfecting piping system of applicant""s invention is comprised of an outer steam pipe and an inner biosolids pipe that runs through the bore of the outer steam pipe. The inner biosolids pipe is configured to have a plurality of spaced apart holes spirally arranged along the periphery of the wall of the inner pipe. These spaced apart holes on the wall on the inner biosolids pipe serve as a plurality of nozzles for injecting a continuous spiraling stream of steam from the outer steam pipe to the biosolids flowing within the inner biosolids pipe.
The spiraling stream of steam created by the plurality of nozzles cause the biosolids flowing through the inner biosolids to rotate or spiral as they flow through the inner biosolids pipe. As a consequence, the spiraling biosolids are mixed with the steam and rotated around within the inner biosolids pipe. This spiraling of the biosolids creates an environment within the inner biosolids pipe that provides for a more even heating of the biosolids with live steam as the biosolids flow through the disinfecting piping system, thus enhancing the disinfecting of the biosolids.
The effectiveness of the disinfecting of the biosolids is dependent upon the amount of time that the flow of biosolids is exposed to the steam treatment. Manipulating the rate of flow of biosolids being pumped though the inner biosolids piping and the pressure of the steam in the outer steam piping can produce a desired treatment time. A computerized control system may be provided to monitor and regulate both steam pressure and pumping rates. Temperatures and pressures can be monitored along the disinfecting piping system and pumping rates and steam pressures can be adjusted to achieve a desired treatment time and temperature of the biosolids.
Ideally, the steam disinfecting piping system is arranged within the biosolids storage tank in a coiled or spiral configuration. The coil configuration serves a means to compensate for expansions and contractions in the piping system due to heat. The heat generated by the steam circulating in the disinfecting piping system also serves to heat the biosolids held in the biosolids storage tank prior to the biosolids introduction into the piping system, reducing heat losses in the disinfecting system.